OV-10 Bronco Replica- Redux

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Staggermania

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Ok,

So I have been looking at the Bronco threads and have done some scaling and crunched some numbers and have come up with the following -

A 3/4 scale Bronco seems to work fairly well.

Stats -

Empty weight - 3000 lbs
MTOW - 4500 lbs
Wing Loading - 27 lbs sqft
Max Speed - 208 kts
Cruise 65% - 170 kts
Stall Clean - 76 kts

Power - 550 hp

Airframe - 2500 lbs
Engines - 1000

Horizontal is about right, verticals about twice what is needed, according to my calcs.
Anyway, thats it for now.:)
 

Staggermania

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So I have been looking at this cut-away, http://static.laodong.com.vn/Uploaded/phamthuhien/2014_12_12/ov (6)_TYBG.jpg , of the Bronco and it appears as if the main spar is around the center of the airfoil, with another spar closer to the front. Is this considered a twin spar arrangement? And if so, would both spars be carrying roughly equal loads?
Also, the Bronco uses a 642A315 airfoil, which I believe is a laminar airfoil. Does anyone have knowledge of this particular airfoil? Would it be a good airfoil for a 3/4 scale Bronc, or would a different airfoil, such as one of Riblett's, be a better choice?

Thanks all.

Brett
 

Autodidact

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Peery discusses how to balance the load between two spars. Getting the spars sized to react proportionally to bending loads leaves the skin or other structure to react to the wings pitching moment (torsion) which would tend to cause a little bit of washout in extreme instances, which is conservative design practice I think? Lots of things going on with the Bronco; pitching loads from the tail, battle damage survivability, who knows what else.
 

Staggermania

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Peery discusses how to balance the load between two spars. Getting the spars sized to react proportionally to bending loads leaves the skin or other structure to react to the wings pitching moment (torsion) which would tend to cause a little bit of washout in extreme instances, which is conservative design practice I think? Lots of things going on with the Bronco; pitching loads from the tail, battle damage survivability, who knows what else.
Thanks AD.
Yeah, a lot going on.Tail loads, lift loads, landing gear, engines. All at the same point. I am waiting on Perry's book. Should be here Tuesday. Lot's of work to do.
 

Staggermania

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So I've been looking at the engine situation. If I am correct, the Bronco's engine weighs in around 300 ish pounds. Adjusted for scale, that means an engine for a 3/4 scale replica would have to come in around 150 lbs or so. Going to be tough to do. For a 275 hp engine, we're looking at 400-500. That means a fair amount of ballast, or moving the engine further back, which would mean a long extension shaft. I think that a single pushed is the practical solution, although it wouldn't be a replica anymore.
Another option would be to put an engine/generator under the wing, with electric motors at the props. Battery packs could be mounted behind the motors in the pots.

Thoughts?
 

Topaz

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...Another option would be to put an engine/generator under the wing, with electric motors at the props. Battery packs could be mounted behind the motors in the pots.

Thoughts?
You'll have both an efficiency penalty and a separate weight penalty. The efficiency of the generator, motor controllers, and the electric motors themselves all pull power away from energy path between the IC motor and the prop. The weight of the additional components adds to whatever a "pure" IC installation would weigh.

If your mission requirements don't preclude the weight penalty, doing a "remote power" installation like this will certainly simplify some aspects of the design. Designing a good system of power conditioning and transmission will complicate it.

Also, a note for your work here: Remember that aircraft weights do not simply scale linearly with the dimensions. The square-cube law comes into play quite distinctly. In this case, I believe it's largely in your favor, but you really can't just scale everything down on an airplane, including the weights, and expect to come up with a viable answer. This really is a complete redesign project, from scratch, and should be treated as such, IMHO.
 

Riggerrob

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Lots of Bronco-replacements have single-pusher propellers in the aft end of the centre fuselage pod. Even the most recent South African prototype has a single engine. An aft-mounted engine would help balance the weight of the front pilot.
 

Staggermania

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You'll have both an efficiency penalty and a separate weight penalty. The efficiency of the generator, motor controllers, and the electric motors themselves all pull power away from energy path between the IC motor and the prop. The weight of the additional components adds to whatever a "pure" IC installation would weigh.

If your mission requirements don't preclude the weight penalty, doing a "remote power" installation like this will certainly simplify some aspects of the design. Designing a good system of power conditioning and transmission will complicate it.

Also, a note for your work here: Remember that aircraft weights do not simply scale linearly with the dimensions. The square-cube law comes into play quite distinctly. In this case, I believe it's largely in your favor, but you really can't just scale everything down on an airplane, including the weights, and expect to come up with a viable answer. This really is a complete redesign project, from scratch, and should be treated as such, IMHO.
Topaz, thanks for the input.
Yes, I am definitely treating this as a new design. I was just scaling factors to get into the ballpark and see if I could spot any major issues, such as the engine mass.
My mission is to build as faithful a replica as I can, so that kind of necessitates IC/generator/motor concept. But a more practical approach would be a single pusher. Perhaps my mission will change as time moves along.
 

Staggermania

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Lots of Bronco-replacements have single-pusher propellers in the aft end of the centre fuselage pod. Even the most recent South African prototype has a single engine. An aft-mounted engine would help balance the weight of the front pilot.[/QUOTE
Thanks Riggerob.

There certainly is merit to the pusher configuration. A nice LS setup from AutoPsru's could certainly get the job done.
 

Staggermania

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So my question now is, in regards to the cg issue with the weight of the engines, ballast or extension shaft?

If I use ballast, I'm thinking it will be somewhere in the neighborhood of 60-80 lbs, about 15' from cg, at the tail. So 30-40 lbs in each boom. Is this realistic? I'm thinking the booms would have to be beefed up just to carry the ballast. If feasible, this might be the simplest solution.

Or, pushing the engine rearward and using an extension shaft, about 20" long.

Which is the lesser of two evils?
 

Topaz

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So my question now is, in regards to the cg issue with the weight of the engines, ballast or extension shaft?

If I use ballast, I'm thinking it will be somewhere in the neighborhood of 60-80 lbs, about 15' from cg, at the tail. So 30-40 lbs in each boom. Is this realistic? I'm thinking the booms would have to be beefed up just to carry the ballast. If feasible, this might be the simplest solution.

Or, pushing the engine rearward and using an extension shaft, about 20" long.

Which is the lesser of two evils?
"I'll take door number three, Alex."

Neither is a good solution, really. The ballast issues compound in that, yes, the booms now have to have extra structure to carry the "useless" ballast mass in all flight conditions and loadings, with flutter issues possibly coming into play as well. Plus there's just the dead extra weight of the ballast itself, sapping performance.

A driveshaft that long (several diameters) is easily into a length where you have to start worrying about torsional resonance, and that's a rabbit hole that you don't want to fall into. Search the forums here and there's plenty of material on the subject that will convince you that you don't want to go there. Plus the prop end of the shaft will need to be supported with two bearings, because you don't want to react gyroscopic loads from the prop with bending in the shaft. More weight, both in the bearings and the structure that supports them.

Take this for what it's worth, but I think you're starting to see the issues that crop up when you try to do an exact scale-up or down from an existing design. The original was optimized for a certain size, weight, and mission, and airplanes just don't scale very well exactly. Far better, IMHO, to work up what the R/C guys call a "stand-off scale" version - one that, from any realistic distance and perspective, looks like an OV-10, but wouldn't stand up to measuring with any precision at all. I think you'll find many of these issues go away with a design that's reminiscent of the real thing, rather than an exact scale copy of the real thing.

YMMV, of course.
 

Staggermania

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"I'll take door number three, Alex."

Neither is a good solution, really. The ballast issues compound in that, yes, the booms now have to have extra structure to carry the "useless" ballast mass in all flight conditions and loadings, with flutter issues possibly coming into play as well. Plus there's just the dead extra weight of the ballast itself, sapping performance.

A driveshaft that long (several diameters) is easily into a length where you have to start worrying about torsional resonance, and that's a rabbit hole that you don't want to fall into. Search the forums here and there's plenty of material on the subject that will convince you that you don't want to go there. Plus the prop end of the shaft will need to be supported with two bearings, because you don't want to react gyroscopic loads from the prop with bending in the shaft. More weight, both in the bearings and the structure that supports them.

Take this for what it's worth, but I think you're starting to see the issues that crop up when you try to do an exact scale-up or down from an existing design. The original was optimized for a certain size, weight, and mission, and airplanes just don't scale very well exactly. Far better, IMHO, to work up what the R/C guys call a "stand-off scale" version - one that, from any realistic distance and perspective, looks like an OV-10, but wouldn't stand up to measuring with any precision at all. I think you'll find many of these issues go away with a design that's reminiscent of the real thing, rather than an exact scale copy of the real thing.

YMMV, of course.
Thanks Topaz.

I get what your saying about the rabbit hole. I like the stand-off scale idea, as I have been thinking that the canopy/fuselage area might need to be a little bit taller/wider. However, in order to make it look like a Bronco, it needs the twin engines, but how do I make that work, and still have it look like a Bronco, from a distance?

I suppose I could move/extend the canopy/fuselage rearward towards the center of the wing, placing the passenger just in front of the cg, and move the pilot a similar amount. This would place the prop just between the pilot and pax.

Perhaps the boom could be shortened somewhat for proportion. Perhaps the prop could be moved closer to the leading edge? What's a good minimum distance?

I'll have to do some sketches.

What would you do, Topaz, if you were designing this?

Brett
 

AIRCAB

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Lots of Bronco-replacements have single-pusher propellers in the aft end of the centre fuselage pod. Even the most recent South African prototype has a single engine. An aft-mounted engine would help balance the weight of the front pilot.[/QUOTE
Thanks Riggerob.

There certainly is merit to the pusher configuration. A nice LS setup from AutoPsru's could certainly get the job done.
How about the Mazda based Rotary engine, high power to wieght !

Steve
 

Topaz

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...What would you do, Topaz, if you were designing this?
Well, I'd probably not be so set on a twin, since I don't have a multi-engine rating and don't see me getting one in any realistic future.

Me, I'd do the single-engine pusher thing, and then keep the "feel" of the Bronco in terms of canopy and tail shapes, wing shape, etc. And then finish it off with a really nice paint job that mimics what the Bronco carried. You can cover a lot of "look and feel" ground with just the shape of a few key items and the paint job.
 

Staggermania

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Well, I'd probably not be so set on a twin, since I don't have a multi-engine rating and don't see me getting one in any realistic future.

Me, I'd do the single-engine pusher thing, and then keep the "feel" of the Bronco in terms of canopy and tail shapes, wing shape, etc. And then finish it off with a really nice paint job that mimics what the Bronco carried. You can cover a lot of "look and feel" ground with just the shape of a few key items and the paint job.
Ok Topaz, here it is -

Bronco.jpg
 

Sockmonkey

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One notion I have regarding small tandem-seat planes like that is to make the rear seat movable on short rails and foldable.
When flying without a passenger move it forward behind the pilot to act as ballast.
When either the pilot or passenger is a big person, you move the rear seat back a bit.
Keeps the CG from changing as much.
 

Staggermania

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One notion I have regarding small tandem-seat planes like that is to make the rear seat movable on short rails and foldable.
When flying without a passenger move it forward behind the pilot to act as ballast.
When either the pilot or passenger is a big person, you move the rear seat back a bit.
Keeps the CG from changing as much.
Great idea Sockmonkey.
I've done some preliminary cg calcs, and the range is about +- 2- 3" on either side of cg, depending on load configuration. The aircraft will lovably have to kneel, however, as empty cg is about 16-18" behind. That or some sort of support extending from the boom/s.
 

Topaz

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Exactly. Someone looks at that, they'll say, "OV-10 Bronco", even though if you put a tape measure to it, the scaled dimensions and proportions aren't really the same.
 
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Staggermania

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Thought I’d resurrect this thread. I had forgotten about this, as life has pulled me in different the past few years.

My question today is, given the configuration as posted in previous drawing. Would this be feasible to do in wood?
As a carpenter, I’m pretty comfortable with wood. I’m thinking wood with fiberglass top layer for smoothness?

Thanks
 

FritzW

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Back in my Marine Corp days ( I was one of Uncle Sam's Misguided Children for 6 years before I joined the USAF and waisted my youth (and middle age) on C-130's :gig:). ...I used to watch OV-10s do touch and goes behind the chow hall. Even back then I thought: "give me a two little 2 stokes, a couple of sheets of 1/8" ply and some Doug Fir and I could make an awesome 1/2'ish scale Bronco.

...it's a pretty cool airplane
 
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